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Fair Value Measurements - Summary of Changes in Fair Value of Company's Level 3 Financial Instruments (Detail) - Derivative Liability - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]    
Balance $ 8,795  
Issuance of convertible bonds with embedded derivative 13,172 $ 8,262
Change in fair value 15,411 533
Conversion of derivative liability to common stock $ (37,378)  
Balance   $ 8,795